Abstract: In this paper, a vertical upward narrow gap rectangular channel is taken as the research object, and the experimental study of single-phase flow characteristics under constant heat flux density of heating conditions is carried out. According to the measured temperature, flow rate, pressure drop and heat flux density, the experimental data of non-isothermal friction factors including laminar, transitional and turbulent flows in a certain range of working conditions are obtained, and the existing empirical correlations are compared and evaluated with the experimental data. The result shows that the predictions of laminar flow friction factor proposed by Kays and Clark and the predictions of turbulent flow friction factors proposed by Blasius, Techo and Moody respectively have a good agreement with the relevant experimental data.